In a wireless network with multiple interfering links, interference alignment (IA) is used. IA is a transmission scheme achieving linear sum capacity scaling with the number of data links, at high SNR. With IA, each transmitter designs the precoder to align the interference on the subspace of allowable interference dimension over the time, frequency or space dimension, where the dimension of interference at each receiver is smaller than the total number of interferers. Therefore, each receiver simply cancels interferers and acquires interference-free desired signal space using zero-forcing (ZF) receive filter.
Most of conventional research on IA is considered to achieve the maximum gain of IA using the infinite selectivity over symbol extensions, which is unrealistic in practical wireless networks. Therefore, the recent studies on MIMO IA focused on the design of IA precoder using a finite space dimension over one transmission slot, which is called MIMO constant channel.
The difficulties of IA in MIMO constant channel is to derive the closed form solution of the IA precoder. Even though the closed form solution in 3 user and some special cases are proposed, it is still unsolved in general case of the number of antennas, user and desired DoF (degree of freedom).
Despite fruitful research and discussion on IA with various aspects, IA is far from being practical. A key challenge for implementing IA techniques is their requirement of perfect and global CSI. This requirement potentially will lead to unacceptable network overhead given the existence of many feedback links and the high-fidelity CSI transmitted over each link.